1. Field of the Invention
This invention relates to photographic apparatus and, more particularly, to electromagnetic driven light controlling components such as camera shutters, aperture stops and the like.
2. Prior Art
U.S. Pat. Nos. 4,024,552; 4,060,313; 4,072,965; 4,121,235; and 4,171,897 disclose shutter mechanisms for photographic cameras in which a pair of shutter blades are driven between positions representing a light blocking or closed condition and a light passing or open condition by forces developed between an electric current conducting coil and a permanent magnet. In the first three of these patents, the shutter blades are each driven in only one direction (shutter opening movement) by the electromagnetic force for return (shutter closing movement) to their initial position by spring means. In the latter two of the cited patents, shutter blade movement in both directions is by electromagnetic force, but the blades are mechanically interconnected by pivot linkage means so that movement of one blade in one direction will result in movement of the other blade in an opposite direction.
In shutter systems of the type represented by the disclosures of the cited patents, the ultimate accurate speed of shutter blade movement is dependent on the magnitude of force available in the electromagnetic driving device and the mass of all components which are moved by such force. In a linear electromagnetic motor of the type represented herein, the force is equal to the product of flux density (B), the length (L) of conductor in the flux path, the electric current (I) in the flux path, and the sine of the angle (.theta.) between the conductor field and the magnetic field. Permanent magnets of rare earth materials such as samarium cobalt have been developed which provide high flux densities in a magnetic field effective at essentially right angles to the magnetic axis of the magnet (sine .theta.=1) particularly when two such magnets are spaced in an iron or otherwise magnetically permeable core with common poles of the two magnets facing each other. Either the coil or the armature in such a motor must be included in the movable components of the system. Because of the need for high density material in the armature and the correspondingly increased mass incident thereto, even in extremely small armature structures, movement of the coil is often preferred to movement of the armature. Such an arrangement is disclosed in U.S. Pat. No. 4,024,552. In this type of linear motor, an effective trade-off can be made in the length (L) of the conductor in the coil and in the size of the coil conductor needed to carry the current (I).
Other movable components in the present system such as the shutter blades themselves and related linkages for their actuation obviously should be kept as light as possible and incur a minimum amount of friction or other resistance to movement in order to minimize the electromagnetic force necessary for proper operation. Since low mass in movable shutter blades has always been an objective, the state of the art relating to shutter blades is presently highly refined. For example, shutter blades currently in use are approximately 150 microns in thickness and each weigh approximately 160 milligrams exclusive of drive mechanisms and linkages. These physical characteristics represent the approximate minimum mass attainable in present blades since sufficient rigidity is required to maintain structural conformation under the stress of compressive forces which are applied during shutter operations. However, prior disclosures such as those found in U.S. Pat. No. 330,775 issued Nov. 17, 1885 and U.S. Pat. No. 2,931,284 issued Apr. 5, 1960, suggest, respectively, double-ended or endless belt structures for shutters which are subjected only to tensile forces. These disclosures, however, do not suggest linear electromagnetic actuation nor minimized resistance to shutter component movement incident to such actuation.